The present invention relates generally to solid-state lasers and, more particularly, to linear frequency-doubled solid-state lasers wherein second harmonic laser light propagates in opposing directions in at least a portion of the laser cavity. Dispersive elements positioned within the laser cavity align the phases of the oppositely-propagating second harmonic laser light beams such that the second harmonic laser light beams are interferometrically added, thereby increasing the output power and efficiency of the laser.
A typical intracavity, frequency-doubled solid-state laser includes a laser medium which lases to produce light at a fundamental wavelength in response to pump radiation generated by a pump source, such as a laser diode. The fundamental laser light, which is usually in the infrared range of the optical spectrum, enters a frequency doubler medium which doubles the frequency to produce second harmonic laser light. Typically, the second harmonic laser light has a wavelength in the visible portion of the optical spectrum. A portion of the second harmonic laser light emanates from the frequency doubler medium and exits through the front end of the laser cavity.
The remaining portion of the second harmonic laser light emanates from the frequency doubler medium in a direction toward the back end of the laser cavity. To increase the efficiency and output power of the laser, it would be advantageous to redirect the backwards-propagating second harmonic laser light toward the front end of the laser cavity. Unfortunately, the redirected second harmonic laser light may destructively interfere with the forward-propagating second harmonic laser light and, consequently, reduce the output power and efficiency of the laser.
In the past, laser designers have avoided this problem by various methods. One method is to permit the backwards-propagating second harmonic laser light to pass out of the back end of the laser cavity. Consequently, the likelihood of destructive interference occurring is practically nonexistent. However, the power contained in the backwards-propagating second harmonic laser light does not contribute to the laser output and, therefore, the laser is relatively inefficient.
Laser designers have also attempted to control the phase of the second harmonic laser light to reduce the possibility of destructive interference in the laser cavity. For example, due to the dispersion of air, the distance between the internal redirection mirror surface and the frequency doubler medium can be carefully varied to control phase. However, as is fully discussed in an article by R. Smith entitled "Theory of Intracavity Optical Second-Harmonic Generation" published in the IEEE Journal of Quantum Electronics, Vol. QE-6, NO. 4, April 1970, at pages 215-223, such a method is impracticable since control of the distance would be required to within a small fraction of a wavelength. Smith also suggests applying an electric field to the frequency doubler crystal or interposing an electrooptic element between the frequency doubler crystal and the output mirror.
Another common method for reducing the potential for destructive interference within the laser cavity consists of changing the polarization of the backwards-propagating second harmonic laser light. An example of such a laser is disclosed in U.S. Pat. No. 5,164,947 issued to Lukas et al. This laser employs a Brewster plate in conjunction with birefringent waveplates positioned between the laser medium and the frequency doubling medium for changing the polarization of the backwards-propagating second harmonic laser light such that the light can be redirected along a path coincident to the forward-propagating harmonic laser light without interference.
Lukas et al. also disclose a laser which utilizes an L-shaped laser cavity wherein the fundamental harmonic laser light is reflected at an angle by a polarizing reflector into the frequency doubler medium. The polarizing reflector is highly reflective at the fundamental wavelength and is highly transmissive at the second harmonic wavelength. After reflection from the polarizing reflector, the fundamental harmonic laser light enters the frequency doubler medium and is converted into second harmonic laser light. A portion of the second harmonic laser light is emitted from the frequency doubler medium through the polarizing reflector to form a portion of the output laser.
The portion of the second harmonic laser light transmitted away from the polarizing reflector is reflected by a dual band mirror through the frequency doubler medium and the polarizing reflector. The phase difference between the second harmonic light emitted toward the polarizing reflector and the second harmonic light reflected from the dual band mirror is determined by the design of the dual band mirror and the distance between the dual band mirror and the frequency doubler medium. To reduce the effect of deviations in this distance, the reflective coating, which is phase-preserving, of the dual band mirror is applied directly to the end of the frequency doubler medium.
Additionally, it has been found that the level of nonlinear coupling between the fundamental and second harmonic laser light within the doubling crystal is dependent upon the amount of second harmonic laser light resonating in the laser cavity. Theoretically, the efficiency of the laser can thus be increased by resonating a greater portion of the second harmonic laser light in the laser cavity. Higher levels of second harmonic laser light in the laser cavity, however, greatly increase the potential for destructive interference and the resulting loss in power and efficiency. Consequently, laser designers have heretofore attempted to limit the amount of second harmonic laser light resonating in the laser cavity.
Accordingly, a need exists in the art for a linear, frequency-doubled solid-state laser wherein the phases of the oppositely-propagating second harmonic laser lights resonating in the laser cavity are controlled such that the second harmonic laser lights constructively interfere to provide increased power and higher efficiency.